In box 5 (r4c4-r6c6), 7 can ONLY go on row 4. So you can eliminate 7 from the other cells in row 4. After this, in box 4 (r4c1-r6c3), 7 can ONLY go in row 5. So you can eliminate 7 from the other cells in row 5. So, eventhough r5c8 started with the possibility of 1 and 7, 7 can be eliminated, leaving only 1.